A very (possibly too) lax WebVTT-to-SubRip converter powered by AWK

vtt2srt.sh

#!/usr/bin/zsh
# vtt2srt: a very (possibly too) lax vtt-to-srt converter
# Copyright (C) 2022 Aleksa Sarai <cyphar@cyphar.com>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# While tools like ffmpeg can convert WebVTT subtitles (.vtt) files to SubRip
# (.srt) if the WebVTT file is correctly formatted, it seems that minor issues
# with the WebVTT cause ffmpeg to give up and output nothing. This is less than
# ideal (especially since it seems that Netflix produces WebVTT files that are
# broken in this way) and ultimately we only need the core bits of information
# from WebVTT in order to produce working SubRip files (in fact, the two
# formats are so similar it's a shame that WebVTT wasn't designed to be a
# backwards-compatible superset of SubRip, though obviously there are reasons
# this wasn't done).
#
# In any case, this is a small awk-powered script which scrapes the minimum
# amount of needed information from the WebVTT file (caring very little about
# the format -- other than the most basic requirement that all segments are
# separated by an empty line) and then outputs an SRT with the same contents
# (with all HTML tags stripped).

set -Eeuo pipefail

[[ "${ZSH_EVAL_CONTEXT:-}" =~ :file$ ]] && IN_SOURCE=1 || IN_SOURCE=

function bail() {
        echo "$@" >&2
        [[ "$IN_SOURCE" ]] || exit 1
}

[ "$#" -eq 2 ] || bail "usage: $0 <vtt-in> <srt-out>"

vtt="$1"
srt="$2"

awk "$(<<-'EOF'
        # After a blank line, the current block has ended.
        /^$/ { in_subtitle = ignore_block = 0 }

        # We ignore all block types (WEBVTT, STYLE, NOTE) because we don't do any
        # styling. Note that this is only done to make sure any weird .vtt files
        # which interleave these with cues are correctly handled.
        /^(NOTE|STYLE|WEBVTT)\>/ { ignore_block = 1 }
        ignore_block { next }

        # If we're in a subtitle line, just output it (stripping out any possible
        # HTML tags. Obviously since we're doing regex, any complicated tags won't
        # work, but who is going to have a field containing ">" in their subtitle
        # file?
        in_subtitle {
                        print gensub(/<[^>]*>/, "", "g", $0)
                        next
        }

        # Only the timing information of a cue is allowed to contain "-->". We only
        # have to change the decimal format (WebVTT uses "." while SRT uses ",").
        # We ignore any of the cue settings since SRT doesn't support them.
        /-->/ {
                        print ""
                        print cue_number++
                        print gensub(/\./, ",", "g", $1), "-->", gensub(/\./, ",", "g", $3)
                        in_subtitle = 1
        }
EOF
)" <"$vtt" >"$srt"